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function woo_custom_add_to_cart($valid, $product_id, $quantity) {
  global $woocommerce;
  $cart = $woocommerce->cart->empty_cart();
  if (!empty($cart) && $valid) { 
    $woocommerce->cart->empty_cart();
    return $valid;
  }
    return $valid;
  }
add_filter('woocommerce_add_to_cart_validation', 'woo_custom_add_to_cart', 10, 3);